The effect of 0-8 wt. % iron on transformations of alpha phase Ni-Mo-Fe alloys with Ni:Mo ratio equal to 4:1

The effect of O-8 wt.% iron in Ni-Mo-Fe alloys of Ni:Mo ratio equal to 4:l is reported using resistivity and hardness measurements, optical, transmission electron and scanning electron microscopy, and X-ray diffraction methods. The research was directed to understanding the transformation of the short-range ordered (sRo) face-centered cubic alphaphase at 850, 800, 700 and 600°C up to 4500 hours. Resistivity measurements were made on heating, cooling, or isothermally for times ranging from minutes to fifiteen hours.

The SRO "K-state" effect exists in the retained alpha-phase of all alloys as shown by a negative temperature coefficient of the resistivity and decrease of resistivity following 55% RA by cold working. The hardness and resistivity of the retained alpha-phase decrease linearly with increasing iron contents. The "critical temperature" of the order-disorder transformation decreases with increasing iron content: 868 ± 2°C for binary Ni4Mo, 86l°C for the 0.35 wt.% Fe alloy, 860°c for the 0.48 wt.% Fe alloy, 848°C for the 0.74 wt.% Fe alloy, 84l°C for the 1.94 wt.% Fe alloy, and 8l5°c for the 3.04 wt.% Fe alloy. The "C-type" curve for the alloys has been determined and established as a function of iron content up to 1.94 wt.%. The nose of the "C-curve" occurs at 775°C/20 sec at 0 wt.% Fe; at 750°C/30 sec at 0.35 wt.% Fe; at 750°C/ 30 sec at 0.43 wt.% Fe; at 747°c/100 sec at 0.74 wt.% Fe; and at 725°c/360 sec at 1.94 wt.% Fe. Based on hardness, resistivity,and optical microstructure, the ordering of the beta-phase in the binary NigMo is completed after aging 10 hours at 850°c, one hour at 800°c, 96 hours at 700°C, and 4500 hours at 600°c.

Individual LRo domains, agglomeration of domains into "walls", and the formation of domain strings are observed within the SR0 matrix in the NigMo composition after aging for one hour at 850°c. An ideal model for these LR0 beta domains is proposed with the habit plane relations (001)β//(00l)α, (il0)s//(i20)α. The domains at 850°C are tetragonal in space with dimensions 2x2xl um. The major morphology is the "walls" of domains along {l00}α planes. A second morphology is strings of domains lying on {lll}α planes and extending in directions ranging from <ll0>α to <l23>α. After 4500 hours at 600°C the structure of the NiqMo composition alloy is predominately uniform domains approximately 150A in diameter. Near grain boundaries, domains enlarge to greater than 5000A by nucleation and growth into the matrix of small domains. LR0 Ni2Mo diffraction spots are also observed from the matrix.

The occurance of an etching effect forming distinct {lll} f.c.c. striations has been correlated to domain size, time and temperature of transformation and iron content. Transformations leading to sufficiently small domain sizes do not develop these striations. Time at temperature results in domain growth; intermediate domain sizes etch to produce striations; largerirregular domains formed after long times again show no striations. However, the internal structure responsible for these striations is not understood. Less distinct {ll0} f.c.c. tapered bands are also found in the beta-phase and in some cases co-exist with {lll} f.c.c. striations.

The solubility of iron in the beta-phase is about 4.5 wt.% at 600°c, 3.2 wt.% at 700°C, 1.3 wt.% at 800°C, and about 0.3 wt.% at 850°C based on heat treatments extending to 4500 hours. A Widmanstatten precipitation of the gamma-phase is found in the alloys with higher iron contents. A secondary coarsening reaction in these alloys to α + Y, α + γ + δ or α + δ is observed near grain boundaries to the extent of about l0% of the structure and emphasizes that equilibrium has not been reached within these times. Distinct co-existence of the alpha- and beta-phase is observed in the Ni-27.03 wt.% Mo, 0.73 wt.% Fe alloy after aging 2000 hours at 850°c. The (1, 1/2, 0) SR0 exists in the f.c.c. matrix. The habit plane of the beta-phase in this condition is {ll0} f.c.c..